The present invention relates to rotary electric machines and more particularly, but not exclusively, to synchronous electric motors having permanent magnets.
In machines having permanent magnets, it is known to use magnetic field detectors in order to know the position of the rotor and to control the excitation of the stator in suitable manner. Known detectors include a Hall effect sensor that is capable of detecting the magnetic field resulting from magnets added to the rotor and therefore distinct from the magnets serving to generate the driving or electromotive force.
The invention may enable to make it easier to build electric machines having permanent magnets.
According to one aspect, the invention provides an electric machine comprising a stator and a permanent magnet flux-concentrating rotor. The rotor may have permanent magnets being engaged between pole pieces. The machine may have at least one magnetic-field detector mounted on the stator in such a manner as to detect the magnetic field of the magnets of the rotor in a location that overlaps a peripheral region of the rotor when the machine is observed on the axis of rotation of the rotor.
Thus, in the invention, it is the magnetic field of the magnets for interacting with the stator that may be detected, such that it may not be necessary to add to the rotor magnets that are intended merely to enable the angular orientation of the rotor to be known. This therefore may simplify manufacture of the rotor.
When the power supply to the motor is polyphase electricity having n phases, the motor preferably includes n detectors fixed on n teeth of the stator, which teeth are advantageously consecutive teeth, thereby facilitating installation of the detectors, access to the detectors, and removal of the electrical conductors connected to the detectors, since all the detectors can be grouped together facing an opening in the case of the motor.
In a particular embodiment, the detectors are fixed on one axial end of the magnetic circuit of the stator and each extends along the axis of a tooth.
In a particular embodiment, each detector includes a Hall effect sensor.
Still in a preferred embodiment, each tooth receives an individual coil which co-operates with the tooth to leave a gap which receives the detector or electrical conductors connected to the detector.
Each individual coil advantageously comprises a bundle of insulated wires, the bundle being substantially flat and wound around a winding axis in such a manner as to form a plurality of superposed turns, the cross-section of the bundle in the superposed turns having a long dimension that extends substantially perpendicularly to the winding axis of the coil. The wires are preferably circular in section, having a diameter that lies in the range 0.3 millimeters (mm) to 2.5 mm, for example. This configuration makes it possible to reduce high frequency losses within the copper at high speeds of rotation of the rotor.
The invention applies more particularly to a motor having a flux-concentrating rotor, the permanent magnets being engaged between pole pieces, the rotor including at least one cheek-plate made of a non-magnetic material, the outside radial edge of the cheek-plate extending slightly set-back from the outside radial edges of the magnets and of the pole pieces in such a manner as to provide an annular region in which the magnetic field can be detected by the above-mentioned detector(s).
Advantageously, the stator has windings on teeth. In a stator with windings on teeth, each tooth serves as the core of a winding. In addition, the number of teeth nteeth is advantageously a function of the number of pairs of poles npairs and the number of phases nphases in compliance with the relationship nteeth=npairs*nphases. 
The combination of a flux-concentrating rotor and a stator with windings on teeth makes it possible to have a machine that is powerful in a small volume, thus making it possible in particular to mount the motor in a cantilevered-out position at the end of a shaft, thereby reducing the number of bearings. The number of teeth and of coils is relatively small, thereby contributing to reducing the cost of the machine.
In addition to a magnetic field sensor, each detector can also include a temperature sensor and the location of the temperature sensor between the coil and the tooth serves to give a good image of the real temperature of the electrical circuit of the corresponding phase.